A steel-making slag is generated through the process of compounding of unnecessary elements which are produced during melting and smelting of metals. Said slag is cooled and crushed thereafter so as to have a preferable grain diameter. Such slag is popularly used as a superior aggregate for road material because it is a hard crystalline.
During the solution and melting operations as mentioned above, unslaked lime (CaO) is used as a dephosphorizing agent and/or a desulfurizing agent, some of which tends to remain and combine with other molecules (hereinafter referred to as free lime). Said free lime expands when it reacts with water. Therefore, when the steel-making slag including such free lime is used as an aggregate for roadbed, it causes a hydration reaction after a certain amount of time elapses, and it produces adverse effects, such as expansion, collapse and deformation of road surface.
A method of combining natural aging and steam aging of steel-making slag has been disclosed in Japanese Laid-Open Patent Publication No. Hei 4-202034. This method, however, requires a preceding natural aging period of one to three months and a steam aging period of about 10 hours. The total processing time is not shortened.
A method of completing the aging of steel-making slag in about one minute by immersing the slag in water and by applying ultrasonic waves to the slag contained in a small container, such as a glass beaker has been disclosed in Japanese Laid-Open Patent Publication No. Hei 5-17182. However, since the slag layer conveyed on a belt conveyor through water is required to be made thicker when the method is applied on an industrial scale, it is difficult to put the method into practical use.
A processing method of using high-temperature steam of 100.degree. C. or more has been disclosed in Japanese Laid-Open Patent Publication No. Hei 6-92696. However, the partial pressure of steam is low since other gases, such as carbon dioxide, are included. Accordingly, since the condensation temperature of the steam is low, the temperature of slag is not raised sufficiently. It cannot be said therefore that the hydration reaction of steam is speeded up sufficiently. In addition, it is indicated that the aging time can be shortened to about four hours by mixing in the carbon dioxide gas. In this method, however, both sources of steam and carbon dioxide are required to be prepared. Furthermore, carbon dioxide gas dissolves in condensed water and the solution becomes acidic, causing the problem of corroding the apparatus used for the method.
A method of aging slag by using water at a pressure of 1.2 kg/cm.sup.2 G or more instead of steam has been disclosed in Japanese Laid-Open Patent Publication No. Hei 6-92697. When water is used as a heating medium, hot water in minute voids between and within the grains of slag is apt to remain when the temperature of the hot water becomes lower than its condensation temperature. Therefore, the hot water in said voids is hardly replaced with higher-temperature hot water, the temperature of which is close to the condensation temperature.
Therefore, the rise in temperature up to the condensation temperature caused by the rise in pressure cannot be fully utilized. As a result, the method requires an aging time of about 12 hours.
A method of charging high-temperature slag having been just discharged from a steel making smelter and having coagulated into a pressure vessel and sprinkling water or the like to perform cooling, aging and crushing simultaneously has been disclosed in Japanese Laid-Open Patent Publication No. Hei 6-127984. In this method, since solids at an extremely high temperature of about 1000.degree. C. must be transferred, the transfer means are required to be heat resistant. Furthermore, since uncrushed large lumps of slag are required to be charged into the pressure vessel, the sealing surfaces of the lids and the walls of the pressure vessel are scratched easily, thereby causing a problem of unsmooth operation.
Therefore, when such slag is used as an aggregate for roads, it is used conventionally after it has been aged so as to be stabilized after completion of expansion and collapse.
Japanese Laid-Open Patent Publication No. Sho 61-101441 discloses that steel-making slag has been conventionally left in natural conditions for three to six months, and proposes a method purportedly capable of significantly reducing the period by blowing high-temperature gas including steam into slag.
However, since slag is processed while it is exposed to the atmosphere, and the high-temperature steam including gas other than steam (air) is blown into the slag in the method, the method still requires a processing time of 48 hours or more.
A method of processing slag in a container such as a hopper has been disclosed in Japanese Laid-Open Patent Publication No. Sho 63-201043. However, since an apparatus, such as a rotary feeder, having relatively low pressure sealing performance is used, the apparatus is not intended for use with high-pressure steam.
Therefore, the slag having been processed in the container of the method is further required to be left out in a natural condition for a certain period. Accordingly, a slag aging period of about two months as required by the conventional method can be reduced only to about a half month.
A method of blowing steam into the slag surrounded at its side and bottom surface by a container has been disclosed in Japanese Laid-Open Patent Publication No. Sho 63-260842. However, since the top of the container is left open or covered with a simple lid, the steam temperature cannot be raised. Therefore, the aging period can be shortened to only about one to three weeks.
A method of performing aging by using saturated steam and warm water while switching two or more slag baths has been disclosed in Japanese Laid-Open Patent Publication No. Hei 3-159938. However, since slag is processed at atmospheric pressure, the temperature of the slag is raised at most up to 80.degree. C. The aging time, therefore, requires about 8 to 24 hours.
A method of performing aging by using high-pressure steam has been disclosed in Japanese Laid-Open Patent Publication No. Hei 4-202033. However, since high-pressure steam is spouted to slag stored at atmospheric pressure, only the enthalpy of the high-pressure steam is utilized, and the temperature of the high-pressure steam is not utilized fully. The aging time therefore cannot be shortened significantly.
Japanese Laid-Open Patent Publication No. Hei 5-238786 applied by one of the applicants of the present invention discloses a method of aging steel-making slag, wherein slag is enclosed in a pressure vessel, steam is supplied into the said vessel to keep the interior of the said pressure vessel under the condition saturated by steam and pressurized higher than atmosphere. According to this disclosure, the aging of steel-making slag is able to be performed only in three hours by maintaining a pressure of about 2 kg/cm.sup.2 G. However, this publication does not disclose any practical methods to conduct the aging of steel-making slag on a large scale, in particular, regarding how to handle the large amount of hot water generated by the condensation of steam which provides latent heat to slag. Therefore, even those skilled in the art have not been able to carry out industrially the aging of steel-making slag under the condition saturated by steam and pressurized higher than atmosphere, although it has the potentiality to shorten by far the time required to age steel-making slag.
The object of the present invention is to provide a method of the aging steel-making slag under the condition saturated by steam and pressurized higher than atmosphere which can be carried out on a large scale industrially, solving the above mentioned problems encountered in the prior art, in particular, regarding how to handle the large amount of hot water generated by the condensation of steam which provides latent heat to slag. To achieve the above mentioned object, the present invention provides an apparatus and a method for aging steel-making slag comprising the steps of; preparing steel-making slag crushed to have a suitable grain size; supplying the said slag into a suitable vessel; supplying steam into the said vessel while discharging the hot condensation of steam in a suitable manner for a suitable period; relieving steam from the said vessel, and; discharging the said slag from the vessel.
Another object of the present invention is to provide a further improved method and apparatus for the aging of steel-making slag under the condition of saturated steam and of pressure higher than atmosphere by adding a preferable method and means for discharging air from the vessel.
Another object of the present invention is to provide a further improved method and apparatus for the aging of steel-making slag under the condition saturated by steam and pressurized higher than atmosphere by adding a preferable method and means for utilizing the heat of the hot condensation of steam discharged from the vessel, which includes a preferable method and means for using a plurality of vessels for the steel-making slag aging.
Another object of the present invention is to provide a further improved method and apparatus for the aging of steel-making slag under the condition saturated by steam and pressurized higher than atmosphere by adding a preferable method and means for discharging air from the vessels when a plurality of vessels is used for the steel-making slag aging.
Another object of the present invention is to provide a further improved method and apparatus for the aging of steel-making slag under the condition saturated by steam and pressurized higher than atmosphere by adding a preferable method and means for eliminating the tiresome work of cleaning the educed calcium hydroxide adhering around the pipe for discharging the hot condensation of steam.
Another object of the present invention is to provide a further improved method and apparatus of the aging steel-making slag under the condition saturated by steam and pressurized higher than atmosphere by adding a preferable method and means for eliminating the dust problem of the steel-making slag discharged from the vessel after the aging.